Use the qmake variable for -ldl on Unix systems
Use the qmake variable for -ldl on Unix systems

There are two important benefits in using this variable:
 * on systems where the library isn't libdl, we now get it right
   (HP-UX, for example, has libdld).
 * on systems where the library isn't necessary, we won't add anything
   (the BSDs, including Darwin)

Change-Id: Ibf8155fd664b426165e55a0b95bf6a2c33dbf1d0
Reviewed-by: Eike Ziller <c142da16c73a95a82d35191bcc55b22835617128@digia.com>
Reviewed-by: Friedemann Kleint <1b9f2592239daaec4856ccc1cfb4d2d55b35fb71@digia.com>
Reviewed-by: Oswald Buddenhagen <6a8868beda3e139b5ac22b2534a23d3e6ba81819@digia.com>

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